1. Field of the Invention
The present invention relates generally to a semiconductor device and its manufacturing method, and particularly to a technique of forming a field oxide film for separating fine pattern elements sensitive to junction leakage such as a dynamic RAM.
2. Description of the Related Art
LOCOS (local oxidation of silicon) is widely used as element isolation technique. However, as devices are scaled down and become progressively smaller, a variety of serious problems have occurred. For example, there occur damages to the surface of a silicon substrate when dry-etching an SiN film which is used as an oxidation mask, formation of a so-called bird's beak in an active region because of a field oxide film creeping under an SiN film at its end portion, and other problems.
Si.sub.3 N.sub.4 film--clad--LOCOS (nitride-clad-LOCOS; NCL) proposed to solve the above problems is a simple and effective Method (e.f. 1993 Symposium on VLSI Technology Dig. Tech. Papers, pp.139-140).
FIGS. 6A to 6E illustrate a method of forming a field oxide film by NCL.
As shown in FIG. 6A, a strain absorbing SiO.sub.2 film 101 is formed on the surface of a silicon substrate 100 to a thickness of about 15 nm. An SiN film 102 is formed to a thickness of 140 nm to 200 nm on the SiO.sub.2 film 101 at the region where a semiconductor device is to be formed. The SiN film is formed by low pressure CVD or the like and patterned by reactive ion etching (RIE).
As shown in FIG. 6B, by using the SiN film 102 as a mask, the SiO.sub.2 film 101 is selectively etched by hydrofluoric acid. At this time, the SiO.sub.2 film 101 is under-etched at the end portion of the SiN film 102 so that a hollow 103 is formed. An overhang 102a of the SiN film 102 is formed over the hollow 103.
As shown in FIG. 6C, the surface of the exposed silicon substrate 100 is thermally oxidized to form an SiO.sub.2 film 104 to a thickness of about 5.5 nm. At this time, the surface of the SiN film 102 is slightly oxidized so that an SiO.sub.2 film is formed thereon.
As shown in FIG. 6D, an SiN film 105 is deposited on the SiO.sub.2 film 104 to a thickness of about 10 nm by low pressure CVD. Because the SiN film is grown generally isotropically and conformably on the exposed surface, the hollow 103 is filled with the SiN film 105. If an n-channel MOSFET is to be formed, impurity ions such as boron are implanted by using the SiN film 102 as a mask to form a channel stop layer 108.
As shown in FIG. 6E, a field oxide film 106 is formed to a thickness of about 500 nm by wet oxidation at a temperature of about 1000.degree. C. During this process, oxidation starts from the surface of the SiN film 105, and after the SiN film 105 on the surface of the silicon substrate 100 has been completely oxidized, the surface of the silicon substrate 100 is oxidized to form the field oxide film 106. The SiN film 105 on the SiN film 102 is also oxidized to form an SiO.sub.2 film 107 combined with the SiO.sub.2 film 104.
However, the SiN film in the hollow 103 is not oxidized because the SiN film 105 covers the SiN film in the hollow 103 and the oxidation speed of an SiN film is very slow as compared to a silicon oxidation speed. Therefore, an SiN island region 105a is left.
As described above, NCL forms the SiN region 105a Just under the end portion of the SiN film 102 which is used as a thermal oxidation mask. Therefore, the oxide film under the SiN region 105a can be made very thin, and oxygen atoms can be prevented from moving under the SiN film 102, thereby suppressing the generation of a bird's beak.
Furthermore, less damages are formed on the surface of the silicon substrate when tile SiN film 102 is removed by reactive ion etching (RIE) because a relatively thick SiO.sub.2 film 101 is formed under the SiN film 102.
As described above, NCL is a method effective for suppressing generation of a bird's beak of a field oxide film and damages to the surface of a silicon substrate.
However, oxidation of the silicon substrate to form a field oxide film starts only after the thin SiN film 105 on the silicon substrate has been completely oxidized. The oxidation speed of an SiN film is very slow and has a high temperature dependency.
A temperature of at least 1000.degree. C. or higher becomes necessary for the SiN film to be completely oxidized in a practical process time. However, at the temperature of 1000.degree. C. or higher, impurities in the channel stop layer 108 formed at the process illustrated in FIG. 6D diffuse more than desired. Diffusion of impurities along the surface of the silicon substrate increases the narrow channel effect which narrows the effective width of a MOSFET channel region.
Use of wet oxidation so as to increase an oxidation speed generates ammonium (NH.sub.3) by a reaction of nitrogen atoms in the SiN films 102 and 105 with hydrogen atoms in water contents. Ammonium molecules diffuse into the SiO.sub.2 films 101 and 104 and reach the surface of the silicon substrate 100. Nitrogen atoms of ammonium react with the silicon substrate at the surface thereof to form SiN. This SiN formed under the SiO.sub.2 film 101 and 104 cannot be removed by the process of etching the SiN films 102 and 105a.
This SiN film functions as an oxidation shielding mask at the later gate oxide film forming process. A stripe region called a white ribbon not thermally oxidized is left at the region near the border of the field oxide film. This white ribbon causes a lowered breakdown voltage of the gate oxide film, and consequently degrades device performance.